1. Field of the Invention
The present invention relates to an image processing apparatus, an automatic focus detecting apparatus, a correction apparatus, a correction method, respectively capable of obtaining a highly precise signal, and to a storage medium storing a program realizing the functions of the apparatuses and method.
2. Related Background Art
Following conventional noise eliminating methods are known.
In an automatic focus detector of a camera or the like, a light beam becomes incident upon a distance measurement optical system and is focussed upon a photoelectric conversion unit with a plurality of line sensors. An output of the photoelectric conversion unit is supplied to a distance measurement unit (generally a microprocessor with a built-in input/output control program for the photoelectric conversion unit and a built-in focus detecting and calculating program) to obtain the focal point.
Noise correction methods for a photoelectric conversion unit usable with an automatic focus detector are described, for example, in Japanese Patent Application Laid-Open Nos. 9-26540 and 10-190038. According to Japanese Patent Application Laid-Open No. 9-26540, a dark current detector is provided for shading a partial area of each of CCD line sensors picking up different portions of an image of a subject to measure an amount of a dark current. The dark current of each line sensor is corrected in accordance with the light amount integration time (accumulation time). According to Japanese Patent Application Laid-Open No. 10-190038, a plurality of load MOS transistors is provided for each output line, the load MOS transistor determining the gain of electric charges of each pixel of an area sensor which charges are inversely amplified and stored. One of the load MOS transistors is selected which, together with a sensor amplifier MOS transistor, makes the fixed pattern noises smallest. In this manner, by devising hardware, the fixed pattern noises are reduced.
Noise elimination for a photoelectric conversion unit usable for picking up an image of a subject is disclosed in Japanese Patent Application Laid-Open No. 6-253217. During the operation of a photoelectric conversion unit, the state while a light beam is not incident and the state while a light beam is incident are alternately repeated. According to Japanese Patent Application Laid-Open No. 6-253217, during the state while a light beam is not incident upon the photoelectric conversion unit, an output of the unit is stored which is subtracted from an output while a light beam is incident. In this manner, dark current components are eliminated.
A dark current is an error signal proportional to the accumulation time and independent from an amount of incidence light upon a photoelectric conversion unit. Fixed pattern noises are noises specific to each pixel of a photoelectric conversion unit and independent from an incidence light amount and accumulation time. FIG. 1 is a schematic diagram showing how dark current and fixed pattern noises change with each pixel of a photoelectric conversion unit and an accumulation time.
Pixels 1 to n shown in FIG. 1 are the pixels of a photoelectric conversion unit. The abscissa represents an accumulation time, and the ordinate represents an error signal of the photoelectric conversion unit. The fixed pattern noise is constant irrespective of whether the accumulation time is long or short. The amplitude of the fixed pattern noise changes with each pixel. The dark current increases as the accumulation time becomes long. The amplitude of the dark current which increases in proportion to the accumulation time changes with each pixel. If the accumulation time is near 0, the dark current of each pixel becomes negligibly small.
FIG. 2 shows a relationship between levels of fixed pattern noises of respective pixels of the photoelectric conversion unit and a threshold level FPNs used as a criterion for judging a defect chip. If the fixed pattern noises are larger than FPNs, the error signal affects the image processing to be executed after the light exposure, and there is a possibility that an image taken with a camera under an AF operation becomes out of focus. A conventional focus detector selectively uses a photoelectric conversion unit having fixed pattern noises smaller than FPNs. Since a variation range of fixed pattern noises of pixels is small, it is sufficient for fixed pattern noise correction that each pixel train with a plurality of pixels of a photoelectric conversion unit is corrected by using the same correction value. For example, it is possible to presume that the fixed pattern noises of pixels shown in FIG. 1 are the same. Therefore, only the dark current is corrected by using different correction values for respective pixels.
Another example of a photoelectric conversion unit usable for taking an image of a subject is shown in FIG. 3. First, the circuit structure of the photoelectric conversion unit will be described. In FIG. 3, reference numeral 51 represents a pixel, reference numeral 52 represents a vertical scanning circuit, reference numeral 53 represents a capacitor for storing a signal combined with an optical signal and a noise signal of each pixel, reference numeral 54 represents a capacitor for storing a noise signal of each pixel, reference numerals 55 and 56 represent transfer MOS transistors for transferring a signal of each pixel to the capacitors 53 and 54, reference numerals 57 and 58 represent transfer MOS transistors for transferring signals stored in the capacitors 53 and 54 to horizontal output lines 59 and 60, reference numeral 61 represents a horizontal scanning circuit, and reference numeral 62 represents a differential amplifier for amplifying a difference between two signals.
Next, the operation of the photoelectric conversion unit will be descried. First, pixels are reset and a noise signal (N) after resetting is transferred to the capacitor 54. Next, the light exposure is performed so that the pixel has a signal combined with an optical signal (S) and a noise signal (N). This signal is transferred to the capacitor 57. The capacitor 57 stores signals S+N and the capacitor 58 stores the signal N. These signals are read to the horizontal output lines 59 and 60. Lastly, the differential amplifier circuit 62 outputs only the light signal S ((S+N)−N).
Such a noise elimination method is also applied to a photoelectric conversion unit with pixels arranged in one-dimension.
Although noises can be eliminated by the above-described methods, these methods may become unsatisfactory for obtaining a highly precise signal.